The U.S. Fire Administration and the National Fire Protection Association (NFPA) estimate that 75% of all deaths, injuries and property damage during a building fire is a direct result of smoke. A natural ventilation cycle occurs in the elevator shaft called "stack effect" drawing smoke into the elevator shaft and exhausting it onto upper floor levels. The taller the vertical shaft and the greater the differential between the inside and outside air temperatures, the greater the draft up the shaft. Historically, elevator systems have dealt primarily with providing a safe means of vertical transportation in multi-story buildings and have not addressed the issue of vertical smoke migration via the hoistway shaft.
The World Trade Center building experienced an explosion and fire within a subterranean parking level. The smoke from the fire migrated through the elevator shafts and within minutes following the explosion caused the evacuation of the entire 110 story building complex. The official report of the NFPA noted the inability of the closed hoistway doors to prevent the migration of the smoke as one of the primary sources of the substantial smoke damage experienced throughout the building.
The basic configuration and operation of an elevator system is well known. A multiple level building contains a vertical shaft defined by a top, bottom and vertical structural walls through which an elevator cab travels between levels. Adjacent to each floor level an opening in the structural wall forms a hoistway entrance through which building occupants can safely pass when the elevator cab is adjacent to the hoistway entrance and registered with the lobby floor. An interlock mechanism connects the elevator car door to the hoistway door when the elevator car is positioned adjacent to a floor such that the elevator car door and the hoistway door are moved together to an open or closed position.
The hoistway entrance comprises a hoistway door head frame attached to a headwall and a pair of hoistway door lateral jambs attached to the jambwall. A sill is displaced below the hoistway door at the floor adjacent to the hoistway entrance opening. A head panel extends from the headwall toward the inner hoistway door to fill the space between the headwall and the hoistway door. The head panel provides an aesthetic shield that blocks the workings of the hoistway door from the view of persons entering or exiting the elevator cab.
Conventional hoistway doors include one or more door panels that are movably supported on a horizontal support rail that is connected to the headwall above the hoistway entrance in a generally horizontal orientation. The doors substantially cover the hoistway entrance opening when they are in the closed position. A clearance gap between the hoistway door and the door frame and between the door panels is necessary to allow the door to open and close without excessive resistance due to contact with the door frame. Movement of the hoistway door panels is restricted to a lateral direction parallel to the hoistway door opening such that the clearance gap is maintained as the hoistway door moves between the open and closed position.
Even though the clearance gap between the elevator hoistway door and the hoistway entrance is limited to 0.375 of an inch by recognized industry standards, large quantities of air freely flow through the clearance gap into and out of the elevator shaft. During a building fire, the stack effect can cause the elevator hoistway to become a smoke stack which quickly distributes smoke and toxic gases throughout the building, thereby jeopardizing human life and property far from the source of the fire.